Historically, most commercially-available, heat-shrinkable, food-packaging films have been polyethylene-based. Polyethylene, particularly ethylene/alpha-olefin copolymers, are relatively inexpensive, have relatively low melting points, are readily heat-sealable, and are readily oriented in the solid state, i.e., at a temperature between the glass transition temperature of the polymer and the melting point of the polymer. Such films can be provided with a relatively high total free shrink value at, for example, 85° C. (185° F.).
In the last few years, semi-crystalline polyamide-based shrink films have begun to compete against polyethylene-based shrink films for the packaging of fresh meat products, even though polyamide is more expensive than polyolefin. One reason is that semi-crystalline polyamide-based shrink films can provide higher impact strength per mil than polyethylene-based films. Unlike patchless bags made from polyethylene-based films, patchless bags made from semi-crystalline polyamide-based films can provide adequate protection for some abusive food products, such as some bone-in meat products.
However, semi-crystalline polyamide-based shrink films are difficult to produce because it is difficult to carry out the solid-state orientation of the semi-crystalline polyamide necessary to impart the desired degree of low-temperature heat-shrinkability. Recently a turn-key production line has been developed and offered for sale by Kuhne Anlagenbau GMBH. Kuhne Anlagenbau GMBH developed this production line concurrently with its development of, and for the manufacture of, a turn-key semi-crystalline polyamide-based multilayer heat-shrinkable packaging film. This Kuhne turn-key production line includes an elaborate quenching means beneath the extrusion die. The quenching means employs vacuum around the exterior of the annular extrudate as it emerges from the die, while at the same time applying water to the extrudate shortly after it emerges from the die. The vacuum is employed so that the annular extrudate does not collapse upon itself before it solidifies enough that it is fully quenched. The vacuum around the outside of the extrudate affects the quenching of the extrudate, because the vacuum draws the applied quenching water from the surface of the extrudate.
While the extrudate is ultimately quenched by the water in the Kuhne process, it would be desirable to quench the extrudate more rapidly, as this could produce an extrudate that is more readily orientable. It would also be desirable to simplify the quenching apparatus. It would also be desirable to improve the reheating and solid state orientation so that the extrudate is more readily oriented in the solid state. It would also be desirable to obtain a film that is more readily orientable.
The multilayer heat-shrinkable film developed by Kuhne has (i) a relatively high semi-crystalline polyamide content that provides the film with relatively high impact strength, (ii) a relatively high total free shrink at 185° F., (iii) a relatively easy-to-seal polyethylene-based heat seal layer containing an easy-to-orient blend containing low density polyethylene blended with linear low density polyethylene.
The Kuhne process also uses specific polyamide blends that are intended to be easier to orient, in order to enable the production of polyamide-based films having high impact strength and relatively high shrink at relatively low temperature. Nevertheless, the solid-state orientation of the extruded tape remains relatively difficult because of its relatively high content of semi-crystalline polyamide. While the low density polyethylene in the seal layer of the Kuhne film facilitates the solid-state orientation of the multilayer polyamide-based extrudate, unfortunately the resulting heat-shrinkable polyamide-based film has exhibited relatively poor optical properties, i.e., a combination of relatively high haze and relatively low transparency. It would be desirable to provide a multilayer heat-shrinkable film containing a relatively high amount of semi-crystalline polyamide, the film having a polyolefin-based heat-seal layer to facilitate heat sealing of the film for the formation of packaging articles such as bags and for sealing the food product, while at the same time providing the film with improved optical properties.
Non-shrinkable retortable pouches have been made from various films containing polymers such as polyethylene, polypropylene, polyamide, and polyester. These non-shrinkable pouches have been made using non-shrinkable retortable films. During retorting, the product to be subjected to retort is surrounded by the non-shrinkable retortable film and placed on a retort rack. Such films need to be capable of withstanding retort conditions and provide high flex-crack resistance and vibration-induced abuse-resistance, without sticking to the retort rack and while maintaining seal integrity. However, products packaged in non-shrinkable films generally have excess film around at least a portion of the perimeter of the product. The result is a packaged product that would be improved by a tighter package with less excess film around the product.
A typical polyethylene-based heat-shrinkable film of the prior art is incapable of withstanding the conditions of retort. Retort conditions are typically from 240° F. to 260° F. for a period of from 10 minutes to 3 hours, under high humidity and high pressure. If a typical heat-shrinkable polyethylene-based film is used to package an article and thereafter subjected to retort, the film shrinks during retort and the resulting strain on the heat seals is so great that the heat seals tend to pull apart during retort. Other heat-shrinkable films that are capable of withstanding elevated temperatures, such as polyamide-containing cook-in films and other films of the prior art, tend to lose seal integrity, delaminate, and/or become embrittled by the retort process, i.e., exhibiting flex-cracking after being exposed to retort conditions. It would be desirable to provide a heat-shrinkable retortable packaging article containing a relatively high amount of polyamide, as for several years, packagers of food products have desired a heat-shrinkable packaging article with good performance in retort end use.